Sheet metal panels are commonly used as components of commercial roofing structures. An assembly of sheet metal panels is fastened together to form a generally flat cover over a roof substrate which may comprise a framework of wood or metal joists, a plywood surface supported on an underlying framework of joists, poured concrete, or the like. Various types of joints are used to fasten the panels into a strong and watertight cover assembly. Standing seam joints comprise a folded connection between adjacent panels which extends vertically upwardly from the panels along the length of the joint. A novel standing seam joint structure is the subject of another patent application of the present inventor. This application pertains to a lap joint which has a primarily horizontal configuration across the joined panels.
A common lap joint structure is that used to assemble the traditional flat lock roof. A flat lock roof panel has edge sections folded back over the main section of the panel to form hemmed edges. The hems are left slightly open to permit hooked engagement with the oppositely facing hem of an adjacent panel to form a joint defined by the overlapping hem sections. The joints are soldered to provide a watertight seal. Although used consistently for many years, this type of joint structure has several problems. For example, the engaged hem sections, when considered in cross section, comprise four layers of sheet metal material which must be thoroughly heated from above to create conditions wherein the molten solder will be drawn into the joint sufficiently to provide a reliable watertight seal. The soldering portion of the assembly process is thus time consuming and skillfully demanding. Soldering problems also arise where the sheet metal panels are nailed or otherwise fastened to the underlying substrate since those punctures through the sheet metal material must be sealed against water. Furthermore, sealing the joints with solder results in a rigid connection between adjoining panels which cannot yield to the strenuous forces induced by thermal expansion and contraction and which may in turn cause buckling of the sheet metal material or breakage of the soldered seal.
Another disadvantage of the traditional flat lock roof joint structure is the difficulty of assembling the panels in an orderly layout along planned lines without accumulating substantial deviations between successively joined edges. This problem is best overcome by assembling a staggered array of panels having a practical size limit of 20.times.28 inches. As the number of joints multiplies with the number of panels, construction of a flat lock roof of any substantial size can become a disproportionately demanding portion of a commercial construction project.
Another type of lap joint structure for a sheet metal roofing assembly may be referred to as a flat lock joint with cleats. Such a joint comprises a row of cleats extending along the length of the joint. The cleats are each nailed or otherwise securely anchored to the roof substrate and include cleat hems interposed between the interlocking hem sections of the sheet metal panels to hold the panels down against the roof substrate. Since the panels are not nailed directly to the underlying substrate but instead are anchored thereto by means of the cleat, this joint structure is superior to the above-described flat lock joint structure which is prone to leak where the anchoring nails perforate the sheet metal panels. However, the overlapping hem sections must still be soldered, and positioning of the cleat hems between the interlocking panel hems brings the number of sheet metal layers which must be thoroughly heated to a total of six. The skill, time, and consequent cost of providing a water tight soldered seal along the entire length of the joint are thereby greatly increased. Furthermore, the panels and cleats are rigidly interconnected through the joint structure and cannot yield to the stress imposed by thermal expansion and contraction of the sheet metal material.
A third type of lap joint structure for a sheet metal roofing assembly consists merely of overlapping panel edges riveted and soldered together. Although this is the strongest type of joint, it, too, suffers from several disadvantages. A simple overlap between panel edges does not accommodate the use of cleats to anchor the panel assembly to the roof substrate, whereby the panels must be anchored by means of nails or other fasteners perforating the panels. Nails not only present an unsightly appearance with frequent damage from hammer blows to the surrounding sheet metal material, but also cause imperfect perforations which are difficult to seal with solder, and their use may be prohibitively labor intensive on a large project. Sheet metal screws are likely to be used more commonly than nails since they may be quickly and easily inserted by means of an automatic driving tool. However, the drilling action of the automatic tool tends to shred the sheet metal material to raise a burr at each perforation which both disrupts the level contour of the panels and increases the difficulty of sealing the perforation with solder. Again, the rigidly anchored assembly cannot accommodate thermally induced movement of the panels.
Methods of constructing known joints for sheet metal roofing assemblies are correspondingly troublesome. Great difficulty is experienced in maintaining adjacent sheets in alignment with a planned layout.
The prior art is thus seen to fail to provide a joint structure for a sheet metal roofing assembly which can easily be soldered without a great deal of time and skill, which accommodates thermally induced movement of the sheet metal panels, and which can be securely anchored to the roof substrate without unsightly and leak-prone perforations through the panels.